fix: Watchdog.shutdownNow() does not shutdown executor

[rgrebski]

Fixes #870 , see my comment

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[rgrebski]

@googlebot I signed it!

[codecov]

Codecov Report

Merging #1158 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1158      +/-   ##
============================================
- Coverage     79.07%   79.07%   -0.01%     
  Complexity     1193     1193              
============================================
  Files           205      205              
  Lines          5258     5257       -1     
  Branches        433      433              
============================================
- Hits           4158     4157       -1     
  Misses          929      929              
  Partials        171      171              

Impacted Files	Coverage Δ	Complexity Δ
...src/main/java/com/google/api/gax/rpc/Watchdog.java	79.50% <ø> (-0.17%)	12.00 <0.00> (ø)

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[igorbernstein2]

lgtm.
Thanks!